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M16C/5L Group, M16C/56 Group
8.3
Clocks Generated by Clock Generators
Clocks generated by the clock generators are described below.
8.3.1
Main Clock
This clock is supplied by the main clock oscillator and used as a clock source for the CPU and
peripheral function clocks. After reset, the main clock is running, but is not used as a clock source for
the CPU.
The main clock oscillator is configured by connecting a ceramic resonator or crystal between pins XIN
and XOUT. The main clock oscillator contains a feedback resistor, which is disconnected from the
oscillator in stop mode in order to reduce the amount of power consumed by the chip. The main clock
oscillator may also be configured by feeding an externally generated clock to the XIN pin.
Figure 8.2 shows Main Clock Connection Example.
MCU
(Built-in feedback resistor)
XOUT
Note:
1. Insert a damping resistor if required. The resistance will vary depending on a crystal/ceramic
Figure 8.2
Main Clock Connection Example
The XOUT becomes high by setting the CM05 bit in the CM0 register to 1 (main clock oscillator turned off)
after switching the clock source for the CPU clock to the sub clock (fC) or on-chip oscillator clock
(fOCO-F, fOCO-S). In this case, the XIN is pulled high to the XOUT via the feedback resistor because the
internal feedback resistor remains connected.
When the main clock oscillator is not used, setting the CM13 bit in the CM1 register to 1 enables to select
the internal feedback resistor not connected.
Perform the following steps to start or stop the main clock. Refer to 8.2 "Registers" for details on register
and bit access.
To start the main clock oscillation:
(1) Set the CM15 bit to 1 (drive capacity high) when a ceramic resonator or crystal is connected
between pins XIN and XOUT.
(2) Set the CM05 bit to 0 (main clock oscillating).
(3) Wait until main clock oscillation stabilizes. (When using an external clock, input the external clock
through the XIN pin.)
To stop the main clock oscillation,
(1) Set the CM20 bit in the CM2 register to 0 (oscillator stop/restart detect function disabled).
(2) Set the CM05 bit to 1 (stop).
(3) Stop the external clock (when inputting the external clock through the XIN pin).
R01UH0127EJ0110 Rev.1.10
Sep 01, 2011
XIN
Ceramic resonator
or crystal
(1)
Rd
VSS
resonator and the oscillation drive capacity setting. Use the value recommended by the
manufacturer of crystal/ceramic resonator.
When the oscillation drive capacity is set to low, check if oscillation is stable in a low state. Also,
place a feedback resistor between XIN and XOUT if the manufacturer recommends placing the
resistor externally.
MCU
(Built-in feedback resistor)
CIN
COUT
XOUT
8. Clock Generator
External clock
XIN
VCC
VSS
Open
Page 104 of 803
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